In history class, the girl to boy ratio is $2$ to $4$. If there are a total of $18$ students, how many girls are there?
A ratio of $2$ girls to $4$ boys means that a set of $6$ students will have $2$ girls and $4$ boys. A class of $18$ students has $3$ sets of $6$ students. Because we know that there are $2$ girls in each set of $6$ students, the class must have $3$ groups of $2$ girls each. There is a total of $6$ girls in history class.